Ion implantation has become a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded in the crystalline lattice of the semiconductor material to form a region of desired conductivity.
The ion source typically generates two or more ion species having different masses. For example, boron produces ion species with atomic masses of 10 and 11. Ion implanters commonly include a mass analyzing magnet and a mass resolving slit for selecting a desired ion species from the ion species generated in the ion source. The analyzing magnet produces a magnetic field which bends the path of each ion species according to its mass. The analyzing magnet also focuses the ion beam in the plane of the mass resolving slit. The magnetic fields are adjusted such that only the desired ion species passes through the resolving slit. The remaining ion species are intercepted by the elements that define the resolving slit and are separated from the ion beam. Ion implanters having mass analyzing magnets and resolving slits are disclosed, for example, in U.S. Pat. Nos. 4,980,562 issued Dec. 25, 1990 to Berrian et al; 4,283,631 issued Aug. 11, 1981 to Turner; 4,118,630 issued Oct. 3, 1978 to McKenna et al and 4,578,589 issued Mar. 25, 1986 to Aitken. An ion beam separator for use in ion implantation is disclosed in U.S. Pat. No. 4,017,413 issued Apr. 12, 1977 to Freeman. The slit disclosed by Freeman is rotatable about an axis parallel with the beam height for adjusting the width of the beam permitted to pass through the slit.
A trend in ion implanters is toward higher beam currents in order to reduce implantation time and thereby increase throughput. When high beam currents are utilized, the current and power density at the mass resolving slit is very high. For beam currents of on the order of 30 milliamps, the power density at the resolving slit may be on the order to 500 to 700 watts per square centimeter (W/cm.sup.2). In a well-tuned beam, only about 20% of the beam power passes through the slit. If the mass resolving slit is cooled by radiation alone, the incident power is sufficient to raise the mass slit temperature to over 2000.degree. C. Even higher temperatures result during the tuning process when the full power density is incident on one side of the slit or the other. Conduction cooling may be used to lower the temperature somewhat. However, both conduction cooling and radiation cooling result in extreme temperature gradients, extending from the edge of the slit into the slit-defining element.
Because of the high operating temperatures, mass resolving slits are usually fabricated of graphite, although refractory metals such as tungsten, molybdenum and tantalum have occasionally been used. All of these materials have a pronounced grain structure. When used as a beam-defining aperture for high current ion implantation, the high thermal stress across the grain boundaries results in fracture and the ejection of a microscopic particle of material. In the case of graphite, amorphous structures such as vitreous carbon reduce the erosion rate, but erosion is still dominated by spalling rather than sputtering, presumably because the vitreous graphites are still inhomogeneous.
The particle ejected from the mass resolving slit experiences a force due to the impinging ions in the ion beam, which impart their momentum to the particle. It can be shown that the force of ion impact on the particle dwarfs the gravitation force for small particles. For reasonable initial velocities, most of the small particles stay in the beam because of the enormous acceleration experienced by ion impact. Larger particles leave the beam more readily because their initial momentum carries them out. Nonetheless, considerable wafer contamination is experienced due to slit erosion in an ion implanter. Contamination of semiconductor wafers being implanted is highly undesirable. Furthermore, the contamination standard has become increasingly strict as the feature sizes of semiconductor devices are reduced.
Another disadvantage of erosion of the mass resolving slit is that the operating life of the slit itself is limited. For high beam currents, the life of the resolving slit may be very short. The resulting downtime for replacement of the resolving slit is costly to the semiconductor manufacturer. Prior efforts at reducing erosion of the mass resolving slit have focused on obtaining better grades of graphite which have high density and therefore have lower thermal gradients and reduced erosion. These efforts have not provided satisfactory results.